The present invention relates to an apparatus and method for continuously annealing a steel strip after cold rolling, wherein the term "steel strip" refers to steel products ultimately applicable to production of cans, steel furniture, automobiles, etc. after continuous annealing, and, if necessary, tin plating, zinc plating, etc.
Generally, on one hand, the higher the tension of a steel strip in a furnace of an apparatus for continuous annealing, the better from the viewpoint of preventing walk or fluttering, and on the other hand, the lower the tension, the better from the viewpoint of preventing heat buckling.
To satisfy such contrasting requirements, it is an ordinary operating practice to decrease the tension of steel strip in a heating furnace or a soaking furnace where the steel strip is at a relatively high temperature and increase the tension in a cooling furnace or an overaging furnace where the steel strip is at a relatively low temperature.
Such tension distribution can be obtained by finely adjusting and controlling a hearth roll speed, and when it is desired to change the tension radically at a position in the furnace, it is an ordinary practice to provide a bridle roll in the furnace.
FIG. 1 shows one example of that practice, where sets of hearth rolls 3 are so arranged in the individual furnaces as to guide a steel strip 1 into the individual furnaces from the inlet side to the outlet side and a set of bridle rolls 2 is arranged just before a rapid cooling (or quenching) furnace 6 in which the steel strip 1 that has passed through a heating furnace 4 and a soaking pit 5 is to be exposed to a gas jet stream of a high speed, and the tension of the steel strip 1 becomes largest after passing through the set of bridle rolls 2. Then, the steel strip 1 passes through the successive quenching furnace 6, an overaging furnace 7 and a final cooling furnace 8 under the largest tension.
The set of bridle rolls 2 and sets of hearth rolls 3 provided for these purposes must reliably restrict the steel strip 1 so as to prevent any slip. Actually, the bridle rolls 2 and hearth rolls 3 have often slipped, particularly when the thickness of the steel strip is small, and have failed to perform their proper functions.
An object of the present invention is to solve these problems and to provide an apparatus for continuously annealing a steel strip, capable of performing functions of bridle rolls and hearth rolls satisfactorily so as to attain a stable operation of the apparatus for annealing.